1. Field of the Invention
The present invention relates to quinoxaline derivatives, and light-emitting elements, light-emitting devices, and electronic devices each including any of the quinoxaline derivatives.
2. Description of the Related Art
By using compounds, compared with inorganic compounds, a wider variety of structures can be taken and materials having various functions depending on the molecular design can be synthesized. Because of these advantages, photo electronics and electronics each using functional organic materials have been attracting attention in recent years.
As examples of electronic devices using functional organic materials, there are solar cells, light-emitting elements, organic transistors, and the like. These are devices using electric properties and optical properties of such organic compounds. In particular, light-emitting elements have been developing remarkably.
A light emission mechanism is said to be as follows: by applying a voltage to a pair of electrodes with a light-emitting layer interposed therebetween, electrons injected from a cathode and holes injected from an anode (hereinafter, electrons or holes are also referred to as carriers) are recombined with each other in an emission center of the light-emitting layer to form molecular excitons, and the molecular excitons release energy in returning to a ground state; accordingly light is emitted. A singlet excited state and a triplet excited state are known as excited states, and light emission is considered to be possible through either a singlet excited state or a triplet excited state.
Such light-emitting elements have a lot of material-dependant problems for improvement of element characteristics. In order to solve the problems, improvement of element structures, development of materials, and the like have been carried out.
The following is known as the most basic structure of light-emitting elements: a hole-transporting layer made of an organic compound having hole-transporting properties, and an electron-transporting light-emitting layer made of an organic compound having electron-transporting properties are stacked to form a thin film of approximately 100 nm thickness in total, and this thin film is interposed between electrodes (Non-Patent Document 1: C. W. Tang et al., Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987)).
By applying a voltage to the light-emitting element described in Non-Patent Document 1, light emission can be obtained from the organic compound having electron-transporting properties.
Further, in the light-emitting element described in Non-Patent Document 1, functions are separated so that the hole-transporting layer transports holes, and the electron-transporting layer transports electrons and emits light. However, various interactions (e.g., exciplex formation) frequently occur at an interface of stacked layers. As a result, changes in the emission spectrum or the decrease in emission efficiency may occur.
In order to suppress the changes in the emission spectrum or the decrease in emission efficiency, which are caused by the interactions at the interface, a light-emitting element having further functional separation has been developed. For example, a light-emitting element has been proposed, in which a light-emitting layer is interposed between a hole-transporting layer and an electron-transporting layer (Non-Patent Document 2: Chihaya Adachi et al., Japanese Journal of Applied Physics, vol. 27, No. 2, L269-L271 (1988)).
In such a light-emitting element described in Non-Patent Document 2, in order to more effectively suppress the interaction occurring at the interface, it is preferable that the light-emitting layer be made of a bipolar organic compound having both electron-transporting properties and hole-transporting properties.
However, most organic compounds are monopolar materials having either hole-transporting properties or electron-transporting properties.
Therefore, bipolar organic compounds having both electron-transporting properties and hole-transporting properties should be developed.
Although a bipolar quinoxaline derivative is described in Patent Document 1 (PCT International Publication No. 2004/094389), its characteristics such as thermal stability are not satisfactory. Thus, a wider variety of bipolar organic compounds should be developed.